Purple Crystal Tiles: Monster Spawner. This tile spawns monsters twice as fast as normal monster spawning. So every time a monster spawns on a normal tile in a dungeon, these tiles spawn 2.

Iron Fences: Gates. They require something to open them. For instance: some require you to disable all spawners in a room or kill a certain amount of monsters. The Boss Room gate requires you to get all four keys in a dungeon and unlock it.

Blue Tiles on wall: Teleporters. Most teleporters you see will be one-way. Meaning that monsters can teleport into the room but you can't teleport into the monster spawn room.

Green Spikes: Spikes. (In the game they won't be green, since the texture was grey Minecraft turned it green.) These spikes kill you instantly, so don't fall into them.

Tiles on the wall that look like they have a hole in them: Launchers. These launchers can either shoot air to push you or a projectile to hurt you. Most of these are seen in Boss Rooms or in Spike Danger Rooms.